I just downloaded latest dokuwiki “Angua” and arctic-mbo template.
In my testing wiki both sidebar and wiki page have link to the wiki page:

[[:test:start|Test]]

The link on the wiki page is wrapped with <span class=“curid”> … </span> while the same link on hte sidebar is not.
I modified lib/tpl/arctic-mbo/layout.css with this code:

/* ---- Current ID highlighting ---- */
.curid {
font-weight: bold;
}

and now only links inside wiki pages are bold, sidebar links are still normal-weighted.

The file inc/parser/xhtml.php is responsible for this functionality. As far as I understand arctic-mbo uses xhtml renderer to present sidebars but apparently it is not working.
Who (dokuwiki or arctic-mbo) is the one to misbehave?

I guess I found the problem. At least it seems so.
Correct me if I'm wrong, but it seems that when dokuwiki parser is parsing the sidebar page, page ID is the sidebar page ID, not the wiki page ID.
I tested it the following way. I inserted the same text into sidebar and a wiki page (they both are in same namespace):

Consequently when I type in my browser http://localhost/wiki/test/sidebar
I see this in the sidebarTestTest sidebar
and this in the wiki page (which is actually the same sidebar just displayed as wiki page)TestTest sidebar